Switch mode power supply (SMPS) may be characterized as small volume, light weight, high power conversion efficiency, etc., and SMPS has been widely used in industrial automation, instruments and meters, medical facilities, liquid crystal display (LCD), communication apparatus, audio-visual products, digital products, and other fields. SMPS is a power supply that utilizes modern power electronics to control a ratio of a switch's on time and off time, so as to remain the output of the power supply substantially steady. Usually, electronic devices, such as MOSFETs controlled by pulse-width modulation (PWM) are used in SMPS circuits.